A. Field of the Invention
The present invention relates to an apparatus and a method for mechanically riveting sheets of metal or the like together and more particularly, to a process in which a rivet is formed as an interference fit within a hole. The present invention finds particular application in the aircraft industry wherein wing skins, fuselage components, etc. are joined together by a plurality of rivets.
Wing skins and other riveted sections in an aircraft are drilled with a plurality of apertures which provide openings for the insertion of rivets which are upset to join the skins together. Such structures in an aircraft are subjected to cyclic loading through the apertures and therefore the fatigue life may be considerably shortened unless the perimeter of the holes is compression stressed. Compression stressing results when a rivet head or button is formed which causes a corresponding radial expansion of the rivet shank against the perimeter of the rivet hole. This swelling of the rivet shank serves as a means for relieving stresses which would be formed adjacent the hole perimeter (area of high stress concentration) during cyclic loading. The present invention contemplates a novel rivet installation procedure by which the rivets are provided with a button at each end, each button formed by a single impact from a high velocity impact gun to cause the rivet shank to swell within a hole.
The present invention incorporates by reference the high energy pneumatically operable impact gun of copending application, Ser. No. 591,972 filed June 30, 1975. The impact gun therein disclosed is employed as the riveting power source of the present invention which requires that the rivet be upset by a single impact.
B. Description of the Prior Art
Prior art riveting methods for producing fatigue-rated rivets in a sheet assembly generally involve squeezing or impacting procedures. Specifically, hydraulic squeezing may be utilized to expand the rivet shank sufficiently in a rivet hole on installation to effect fatigue life of the assembled structure. However, hydraulic squeezing involves considerable initial monetary investment in the squeezing machine and further requires time-consuming positioning of large, cumbersome wing skins of the like.
A method involving simultaneous impacts has been developed previously by the Boeing Co. (U.S. Pat. No. 3,559,269). This method uses a high energy single impact gun known as the Electro Magnet Riveter (EMR). The EMR gun also requires a high initial investment and a high operation expense due to short coil life and high cost of coils. Furthermore, the EMR must be suspended by a counterbalance system and is therefore not truly portable.
Another method to effect fatigue rating is to cold work a rivet hole with special cold-working equipment prior to installation of a rivet. Such a method is considered less effective and is used as a secondary process only when it is not feasible to employ the above mentioned procedures. Cold-working is time consuming and requires extra man hours as well as additional equipment.
In U.S. Pat. No. 3,609,851, there is disclosed a metal working apparatus and process in which an electromechanical transducer applies energy to a work piece for the deformation of metallic or non-metallic materials in procedures such as riveting, cold heading, bonding and swaging. In one disclosed embodiment, electromechanical energy is applied to a metal insert by downward force of an impact tool. The impact forces the metal insert downward until its extreme end abuts against a recess formed in a back-up mass. By continuing impact, a metal rivet will be deformed until a rivet head is formed between a countersink and a depression. The next step requires that the plate assembly which is to be riveted together be reversed and that the source of static force also be inverted in order that the depression will receive the rivet head previously formed by the cup-shaped depression. Impact is once again continuously applied until the previously formed rivet head seats itself in the depression and the metal rivet deforms on its opposite end until a second rivet head is formed by the depression.
A high impact portable riveting apparatus is disclosed in U.S. Pat. No. 3,559,269. Here, rapidly changing electrical current passing through a coil is converted into a mechanical force acting between a conductor plate on a ram and the coil to propel the ram and its forming surface against a workpiece. A pair of portable electromechanical riveting guns are described along with the electrical system and are interlocked in their operation to insure a simultaneous ram impact on each end of a rivet to be upset. While the system disclosed in this patent provides for the forming of a button on each end of a rivet shank, it can be appreciated that the system is expensive and cumbersome, in that heavy guns are required which utilize an electrical motive source. On the other hand, the present invention provides a process in which a pneumatically operable system is employed to form a button on one end of a rivet shank and then form a button on the other end.
Other patents of relevant interest but not deemed worth of further comment are U.S. Pat. Nos. 2,753,624, 3,135,141 and 3,562,893.